Gas velocity patterns in simulated galaxies: Observational diagnostics of spiral structure theories

TL;DR

This study compares gas velocity patterns in simulated galaxies to observationally distinguish between long-lived steady spiral arms and transient dynamic spiral arms, revealing key differences in gas flow directions that can serve as diagnostics.

Contribution

It identifies specific gas velocity pattern differences predicted by the two spiral theories, providing observational diagnostics to discriminate between steady and dynamic spiral models.

Findings

Steady spiral arms show radial streaming motions inside co-rotation radius.

Dynamic spiral arms tend to have tangential streaming motions.

Gas velocity patterns alone are insufficient to confirm spiral arm longevity.

Abstract

There are two theories of stellar spiral arms in isolated disc galaxies that model stellar spiral arms with different longevities: quasi-stationary density wave theory, which characterises spirals as rigidly rotating, long-lived patterns (i.e. steady spirals), and dynamic spiral theory, which characterises spirals as differentially rotating, transient, recurrent patterns (i.e. dynamic spirals). In order to discriminate between these two spiral models observationally, we investigated the differences between the gas velocity patterns predicted by these two spiral models in hydrodynamic simulations. We found that the azimuthal phases of the velocity patterns relative to the gas density peaks (i.e. gaseous arms) differ between the two models, as do the gas flows; nevertheless, the velocity patterns themselves are similar for both models. Such similarity suggests that the mere existence of streaming motions does not conclusively confirm the steady spiral model. However, we found that the steady spiral model shows that the gaseous arms have radial streaming motions well inside the co-rotation radius, whereas the dynamic spiral model predicts that the gaseous arms tend to have tangential streaming motions. These differences suggest that the gas velocity patterns around spiral arms will enable distinction between the spiral theories.